The electrical stimulation of nerves, often afferent nerves, to indirectly affect the stability or performance of a physiological system can provide functional and/or therapeutic outcomes, and has been used for activating target nerves to provide therapeutic relief of pain.
While existing systems and methods can provide remarkable benefits to individuals requiring therapeutic relief, many issues and the need for improvements still remain.
Many techniques have been developed to treat pain, but all of them are ultimately insufficient.
Non-narcotic analgesics, such as acetaminophen or non-steroidal anti-inflammatory drugs (NSAIDS), have relatively minor side effects and are commonly used for several types of pain. However, they are rarely sufficient in managing moderate to severe chronic pain (Sherman et al. 1980; Loeser 2001a; Rosenquist and Haider 2008).
The use of narcotic analgesics, such as N-methyl-D-aspartate (NDMA) antagonists, has shown only minor success with inconsistent results. Narcotics carry the risk of addiction and side effects, such as nausea, confusion, vomiting, hallucinations, drowsiness, dizziness, headache, agitation, and insomnia.
Psychological strategies, such as biofeedback and psychotherapy, may be used as an adjunct to other therapies but are seldom sufficient, and there are few studies demonstrating efficacy.
Electrical stimulation systems have been used for the relief of pain, but widespread use of available systems is limited.
There exist both external and implantable devices for providing electrical stimulation to activate nerves and/or muscles to provide therapeutic relief of pain. These “neurostimulators” are able to provide treatment and/or therapy to individual portions of the body. The operation of these devices typically includes the use of an electrode placed either on the external surface of the skin and/or a surgically implanted electrode. In most cases, surface electrode(s), cuff-style electrode(s), paddle-style electrode(s), spinal column electrodes, and/or percutaneous lead(s) having one or more electrodes may be used to deliver electrical stimulation to the select portion of the patient's body.
Transcutaneous electrical nerve stimulation (TENS) has been cleared by the FDA for treatment of pain. TENS systems are external neurostimulation devices that use electrodes placed on the skin surface to activate target nerves below the skin surface. TENS has a low rate of serious complications, but it also has a relatively low (i.e., less than 25%) long-term rate of success.
Application of TENS has been used to treat pain successfully, but it has low long-term patient compliance, because it may cause additional discomfort by generating cutaneous pain signals due to the electrical stimulation being applied through the skin, and the overall system is bulky, cumbersome, and not suited for long-term use (Nashold and Goldner 1975; Sherman 1980; Finsen et al. 1988).
In addition, several clinical and technical issues associated with surface electrical stimulation have prevented it from becoming a widely accepted treatment method. First, stimulation of cutaneous pain receptors cannot be avoided resulting in stimulation-induced pain that limits patient tolerance and compliance. Second, electrical stimulation is delivered at a relatively high frequency to prevent stimulation-induced pain, which leads to early onset of muscle fatigue in turn preventing patients from properly using their arm. Third, it is difficult to stimulate deep nerves and/or muscles with surface electrodes without stimulating overlying, more superficial nerves and/or muscles resulting in unwanted stimulation. Finally, clinical skill and intensive patient training is required to place surface electrodes reliably on a daily basis and adjust stimulation parameters to provide optimal treatment. The required daily maintenance and adjustment of a surface electrical stimulation system is a major burden on both patient and caregiver.
Spinal cord stimulation (SCS) systems are FDA approved as implantable neurostimulation devices marketed in the United States for treatment of pain. Similar to TENS, when SCS evokes paresthesias that cover the region of pain, it confirms that the location of the electrode and the stimulus intensity should be sufficient to provide pain relief and pain relief can be excellent initially, but maintaining sufficient paresthesia coverage is often a problem as the lead migrates along the spinal canal (Krainick et al. 1980; Sharan et al. 2002; Buchser and Thomson 2003).
Spinal cord stimulation is limited by the invasive procedure and the decrease in efficacy as the lead migrates. When it can produce paresthesias in the region of pain, spinal cord stimulation is typically successful initially in reducing pain, but over time the paresthesia coverage and pain reduction is often lost as the lead migrates away from its target (North et al. 1991; Andersen 1997; Loeser 2001a).
Lead migration is the most common complication for spinal cord stimulators occurring in up to 45-88% of the cases (North et al. 1991; Andersen 1997; Spincemaille et al. 2000; Sharan et al. 2002). When the lead migrates, the active contact moves farther from the target fibers and loses the ability to generate paresthesias in the target area. SCS systems attempt to address this problem by using leads with multiple contacts so that as the lead travels, the next contact in line can be selected to be the active contact.
Peripheral nerve stimulation may be effective in reducing pain, but it previously required specialized surgeons to place cuff- or paddle-style leads around the nerves in a time consuming procedure.
These methods of implementation have practical limitations that prevent widespread use. External systems are too cumbersome, and implanted spinal cord stimulation systems often have problems of lead migration along the spinal canal, resulting in either the need for frequent reprogramming or clinical failure.
Percutaneous, intramuscular electrical stimulation for the treatment of post-stroke shoulder pain has been studied as an alternative to surface electrical stimulation. A feasibility study (Chae, Yu, and Walker, 2001) and a pilot study (Chae, Yu, and Walker, 2005) showed significant reduction in pain and no significant adverse events when using percutaneous, intramuscular electrical stimulation in shoulder muscles.
This form of percutaneous, intramuscular electrical stimulation can be characterized as “motor point” stimulation of muscle. To relieve pain in the target muscle, the percutaneous lead is placed in the muscle that is experiencing the pain near the point where a motor nerve enters the muscle (i.e., the motor point). In “motor point” stimulation of muscle, the muscle experiencing pain is the same muscle in which the lead is placed. In “motor point” stimulation of muscle, the pain is felt and relieved in the area where the lead is located.